Notch is a transmembrane receptor involved in cell fate decisions in diverse organisms ranging from Drosophila to humans. Recently, signaling through Notch has been implicated in multiple cell fate decisions in lymphocyte development. Despite the evidence supporting a role for Notch, how Notch activity is regulated during lymphocyte development is not known. One of the proteins implicated in the regulation of Notch is Numb. Numb is an adapter protein that binds to Notch and inhibits its activity. The major aim of this proposal is to examine the role of Numb in lymphocyte development. To achieve this we will use several complementary approaches. Specific aim 1 will test the effect of overexpression of Numb and a dominant negative form of Numb in T cell lineage determination. Specific Aim 2 will test the effect of Numb deficiency in T cell lineage determination in vivo using a thymocyte specific conditional Numb knockout. Lastly, specific Aim 3 will test the role of Numb in B versus T cell lineage determination.